A method for calculating correlation between a first and a second code sequence, which are previously known code sequences used in telecommunications systems, comprise a number of samples and have a finite length, and in which method the delay between the code sequences is calculated and the codes are set to a desired phase with respect to each other on the basis of the delay between the code sequences by rotating the first sequence.
The present invention can be used for calculating auto/crosscorrelations between finite length sequences. The disclosed method and arrangement are applicable to any situation in which said correlations are to be calculated. The method can be particularly preferably applied to data transmission systems utilizing a direct sequence code division multiple access (DS-CDMA) method, in which systems correlations need to be calculated between differently delayed codes.
In the DC-CDMA the user""s narrow-band data signal is modulated to a relatively wide band with a spreading code having a broader band than the data signal. In the used systems band widths from 1.25 MHz to 50 MHz have been employed. A spreading code usually consists of a long pseudo-random bit sequence. The bit rate of the spreading code is much higher than that of the data signal, and to distinguish it from data bits and data symbols the bits of the spreading code are called chips. The data symbol of each user is multiplied by all chips of a spreading code. The narrow-band data signal thus spreads to the frequency band utilized by the spreading code. Each user has a spreading code of his own. Several users simultaneously transmit over the same frequency band, and data signals are distinguished from one another in the receivers on the basis of the pseudo-random spreading code. However, different users""signals interfere with one another in the receiver, since the spreading codes are not completely orthogonal especially due to the phase shift caused by propagation delay.
In the CDMA systems the multiple access interference is reduced e.g. by means of multi-user detection (MUD) or interference cancellation methods. There are several such methods, but it is common to them that they need input information on the correlations between the outputs of the received signal.
According to the prior art, correlations between finite length sequences are typically calculated in serial mode one symbol at a time by using shift-register-based arrangements and by transmitting the sequence in a register forward between the calculations. The most significant drawback of the serial mode processing is its slowness, which becomes particularly apparent in telecommunications applications.
An object of the present invention is to provide a method for calculating correlation, which allows calculation to be performed quickly with a simple arrangement.
This is achieved with the method described in the introduction, characterized in that the correlation between the sequences is calculated by comparing the sequences with each other sample by sample in parallel mode, and by adding the results of the sample-specific comparisons together.
The invention further relates to an arrangement for calculating correlation between a first and a second sequence, which are previously known code sequences used in telecommunications systems, comprise a number of samples and have a finite length, and which arrangement comprises means for estimating the delay between the code sequences and means for setting the codes to a desired phase with respect to each other on the basis of the delay between the code sequences by rotating the first sequence. The arrangement of the invention is characterized in that the arrangement comprises means for comparing the sequences with each other sample by sample in parallel mode, and means for adding the results of the sample-specific comparisons together.
The method and arrangement of the invention have several advantages. The correlation calculation implemented in parallel mode according to the invention is fast. The method can also be applied to cases in which the sequences to be correlated are of different lengths. When the invention is applied in telecommunications systems, the users""transmission rates may differ from one another.